Plural oscillator system for generating simultaneous pairs of sequential tones

ABSTRACT

A first oscillator is rendered operative by a pulse to produce a first tone of limited duration. A second oscillator has frequency determining elements which are sequentially coupled in circuit by a sequence of pulses applied to the second oscillator, to produce a sequence of second tones in time coincidence with the first tone.

[ June 18, 1974 United States Patent [191 Wycoff PLURAL OSCILLATORSYSTEM FOR 3 513 399 5/1970 Wycoff. GENERATING SIMULTANEOUS PAIRS3.597,690 8/1971 Wycoff [76] Inventor:

Keith H. Wycoff, P.O. Box 308,

Primary Examiner-Herman Karl Saalbach Assistant ExaminerSiegfried H.Grimm Attorney, Agent, or Firm-Prangley, Dithmar, Vogel, Sandler &Stotland O O0 O0 6 L w 3 N7 9 n1 oo .m b e .LF .m k F 2 2 rt 21 Appl.No.: 336,779

A first oscillator is rendered operative by a pulse to produce a firsttone of limited duration. A second oscillator has frequency determiningelements which are sequentially coupled in circuit by a sequence ofpulses applied to the second oscillator, to produce a sequence of secondtones in time coincidence with the first tone.

, M2Mm 2 2 2 /3 3 63 4 54 3 S; R 54 .,6 a 5 ,5 H WW5 ,1 M32 U m 3 m 36"M mm m 3 H m5 ."m n n .c m mm m ne M u 1.8 S WM U .mw We H mM 5 [5 6]References Cited UNITED STATES PATENTS 3,204,045 8/1965 Tuthill et a].179/41 A 3 Claims, 5 Drawing Figures MIXER R F il AMP 25 2e l I I. F

FlLTER AMP BALANCED MC DULATOR AUDIO AMP 'T I I42 PATENTED 3.818.368

SHEET 2 BF 4 FIG. 2

PLURAL OSCILLATOR SYSTEM FOR GENERATING SIMULTANEOUS PAIRS OF SEQUENTIALTONES This is a division, application Ser. No. 165,475, filed July 26,1971, now US. Pat. No. 3,771,060.

It is an important object of the present invention to provide animproved oscillator for use in generating simultaneous pairs ofsequential tones.

In summary, there is provided a first oscillator for generating a firsttone, first means for producing a first pulse of limited duration, thefirst oscillator being coupled to the first means and responsive to thefirst pulse to produce the first tone for the duration of the firstpulse, a second oscillator having frequencydetermining elements andbeing operative to produce second tones having frequencies respectivelyin accordance with the frequency-determining elements in circuit saidthe second oscillator, second means for producing a sequence of aplurality of second pulses respectively of limited durations, the secondoscillator being coupled to the second means and responsive to thesecond pulses to produce a sequence of a plurality of second tonesrespectively for the durations of the second pulses, means coupled tothe outputs of the first and second oscillators for combining the tonestherefrom and thereby provide a sequence of a plurality of second tonesand a continuous first tone simultaneously with the plurality of secondtones.

With the foregoing and other objects in view, which will appear as thedescription proceeds, the invention consists of certain novel featuresand a combination of parts hereinafter fully described, illustrated inthe accompanying drawings, and particularly pointed out in the appendedclaims, it being understood that various changes in the details of thecircuitry may be made without departing from the spirit or sacrificingany of the advantages of the invention.

For the purpose of facilitating an understanding of the invention, thereis illustrated in the accompanying drawings a preferred embodimentthereof, from an inspection of which, when considered in connection withthe following description, the invention, its mode of construction,assembly and operation, and many of its advantages should be readilyunderstood and appreciated.

FIGS. 1 and 2 illustrate a transmitter, partially in block and partiallyin schematic, used in the communication system incorporating thefeatures of the invention;

FIG. 3 is a block diagram of the receiver in the systern;

FIG. 4 is a schematic diagram of the tone extractor forming part of thereceiver of FIG. 3; and

FIG. 5 is a schematic of the decoder and the electronic switch of thereceiver of FIG. 3.

Referring now to the drawings, and more particularly to FIG. 1 thereof,there is shown a single side-band transmitter for transmitting singleside-band signals with a suppressed carrier. The transmitter 20 includesan audio amplifier 21 for applying an audio signal to a balancedmodulator 22, the modulator 22 having a second input to which is appliedan oscillatory signal derived from a first oscillator 23. The balancedmodulator 22 mixes the audio signal (the modulation frequencies) and theoscillatory signal (the carrier wave), and passes the sum and differencefrequencies. The modulation frequencies are attenuated substantiallybecause of the band-pass characteristics of the modulator 22, and thecarrier wave is balanced out electronically. The modulation componentsmay either be in the form of a voice message applied to the audioamplifier 21 by way of the microphone 24 or from a tone generator to bedescribed in detail hereinafter.

The upper and lower side bands produced in the balanced modulator areapplied to a filter 25 which passes only a selected one of the sidebands, the selected side band being amplified in an IF amplifier 26. Theamplified IF signal is applied to a mixer 27 which also receives ahigher frequency, second oscillatory signal from the second oscillator28, thereby to provide a modulated signal at radio frequencies. The RFsignal is amplified in an RF amplifier 29 and is radiated by an antenna30. The elements just described are elements well-known in the art, sothat further description thereof is unnecessary. Also, it is to beunderstood that any suitable alternative transmitter capable of singleside-band transmission is contemplated.

Turning now to the tone generator 40, there is provided a power supplyhaving a Zener diode 47 and a resistor 46 coupled in series therewith toan A+ source of supply voltage, which may be a battery, for example.Accordingly, a reduced B+ supply voltage is available across the Zenerdiode 47. In one form of the invention, the A+ supply voltage was 12volts and the Zener diode was of 9-volt variety, so that the B+ supplyvoltage was 9 volts. Also, the tone generator 40 has a switch 41, which,in the form shown, is depressible and has a normally-open condition, theswitch 41 being coupled between ground reference potential and resistor42, to a first switch circuit 50. The first switch circuit 50 includes afirst NPN transistor 51, with a resistor 52 coupled between thecollector and base thereof. A diode 53 is coupled between the emitterand the base of the transistor 51, the collector thereof being coupledthrough a resistor 54 to the A+ supply voltage. The emitter of thetransistor 51 is coupled byway of a Zener diode 55 to the base of asecond NPN transistor 56, the collector of which is coupled through aresistor 57 to the A+ supply voltage, and the emitter of which iscoupled by way of a resistor 58 to ground reference potential. Aresistor 59 is coupled between the base and emitter of the transistor56. A third NPN transistor 60 has its base coupled to the collector ofthe transistor 56, its collector coupled to the A+ operating voltage bymeans of a resistor 61, and its emitter coupled through a diode 62 tothe collector of a fourth NPN transistor 63. The base of the transistor63 is coupled to the junction of the'resistors 58 and 59, and theemitter is coupled to ground reference potential.

When the power supply 45 is energized to provide the A+ supply voltagefor the first switch circuit 50, the transistors 51, 56, and 63 are allrendered conductive, thereby effectively to ground (except for thesaturation resistance between the collector and emitter) the collectorof the transistor 63. The transistor 56 shunts current away from thetransistor 60, whereby the latter is rendered non-conductive. When theswitch 41 is actuated, the base of the transistor 51 is effectivelygrounded through the resistor 42, thereby shunting less current awayfrom the transistor 51 and rendering it non-conductive, which, in turn,renders nonconductive the transistors 56 and 63. Since the transistor 56is no longer conductive, current will flow through the resistor 57 intothe transistor 60, thereby rendering the same conductive and causing theA+ voltage, minus the small drop across the resistor 61, the transistor60, and the diode, to appear on the collector of the transistor 63.Summarizing, if no voltage is applied to the input, that is, to the baseof the transistor 51, the output voltage of the switch circuit 50 willbe zero, that is, the collector of the transistor 63 will be on ground.For convenience, in future reference, the grounded condition of theoutput of a switch circuit will be referred to as yielding a zerovoltage despite the fact that the voltage is somewhat higher because ofthe saturation resistance of a transistor. Similarly, a positive voltageat the input of the switch circuit 50 will cause an increase inconduction of the transistor 51, but the output voltage will still bezero. If the input to the switch circuit 50 is grounded, that is, if azero voltage is applied thereto, the output of the switch circuit 50will be a positive voltage slightly less than the A+ voltage. Thus, itmay be seen that the switch circuit 50 may be considered an inverter,that is, if the input if zero voltage, the output will be a plusvoltage, and, if the input is a plus voltage, the output will be zerovoltage.

The output of the first switch circuit 50, that is, the collector of thetransistor 63 is coupled by way of a resistor 65 to a second switchcircuit 70. Also coupled to the input of the second switch circuit 70 isa resistor 67 coupled in series with a normally-open switch 66 to the A+supply voltage. The second switch circuit 70 has precisely the sameconstruction as the first switch circuit S0, and the parts are thereforenumbered with corresponding numbers, but with added thereto. in theinterest of simplifying the drawing, only the input and output of theswitch circuit 70 are illustrated.

Prior to actuating the switch 41, the collector of the transistor 63 iseffectively grounded, so as to divert current from the transistor 71 inthe second switch circuit 70. Thus, the transistors 71 and 83 will berendered nonconductive, and the output of the second switch circuit 70will be a plus voltage. Actuation of the switch 41, to ground the inputof the first switch circuit 50, causes the output thereof to provide theplus voltage, as previously explained, which causes the transistors 71and 83 to conduct, thereby grounding the output of the second switchcircuit 70, so that it supplies a zero voltage. The same result can beaccomplished by actuating the switch 66, which would apply the A+voltage directly to the transistor 71, thereby rendering it and thetransistor 83 conductive, to cause a zero voltage to appear at theoutput of the second switch circuit 70. Thus, actuation of either of theswitches 41 or 66 has the same net effect of providing a zero voltage inthe output of the second switch circuit 70.

The output of the second switch circuit 70, that is, the collector ofthe transistor 83, is coupled by way of a capacitor 85 to a third switchcircuit 90. The switch circuit 90 has the same construction as theswitch circuit 50 and has corresponding numbers, but with 40 added.Again, in the interest of simplifying the drawing, there are shown onlythe input and output stages of the switch circuit 90.

Before either of the switches 41 or 66 is actuated, the transistors 91and 103 are conducting, so that the output of the third switch circuit90 is a zero voltage. When either of the switches 41 or 66 is actuated,the transistor 83 in the second switch circuit 70 becomes heavilyconductive, to cause current flow through the resistors 92 and 94 to bediverted from the transistor 91 and through the capacitor and thecollector and emitter of the transistor 83. Accordingly, the transistor91 is rendered nonconductive as is the transistor 103, to cause theoutput of the switch circuit to rise to the plus voltage. When thecapacitor 85 has become charged, further current flow through theresistors 92 and 94 is applied to the transistor 91 to render the sameconductive and therefore render the transistor 103 conductive.

Thus, a positively-directed pulse 104 appears at the output at the thirdswitch circuit 90, that is, on the collector of the transistor 103. Theleading edge of the pulse 104 occurs at the time that the transistor 83becomes conductive, which occurs essentially simultaneously withactuation of either of the switches 41 or 66. The capacitor 85 and theresistors 92 and 94 define a time constant network, the values of whichcontrol the duration of the pulse 104.

The output of the third switch circuit 90 is coupled to a fourth switchcircuit which has a construction identical to the switch circuit 50,with corresponding numerals, but with 60 added thereto. Again, forsimplicity purposes, only a portion of the switch circuit 110 isillustrated. When the collector of transistor 103 in the third switchcircuit 90 is at the plus voltage for the duration of the pulse 104, thetransistor 111 and, thus, the transistor 123 are conductive, effectivelygrounding the collector of the transistor 123, thus furnishing anegatively-directed pulse 124 on the conduc tor 125. The conductor 125is coupled to a splitter circuit 130, which circuit includes anisolating resistor 131 coupled to the base of a PNP transistor 132. Theemitter of the transistor 132 is coupled to the B-lsupply voltage andthe collector is coupled to a first oscillator 190. There is providedanother isolating resistor 134 coupled to the conductor 125 and a secondPNP transistor 135 having its base coupled to the resistor 134. Thecollector of the transistor 135 is coupled to a second oscillator 210,which will be described presently. The negatively-directed pulse 124renders the transistors 132 and 135 conductive so as to furnish upon therespective collectors positively-directed pulses 136 and 137 havingdurations equal to the duration of the pulse 124.

The output of the third switch circuit 90 is also coupled to a fifthswitch circuit 140 by way of a capacitor 139, which switch circuit 140is substantially identical to the switch circuit 50 and is labeled withcorresponding numbers, but with 90 added thereto.

Prior to the appearance of the pulse 104, the transistors 141 and 153are conducting, so that the output of the fifth switch circuit 140 is azero voltage. The leading edge of the pulse 104 from the third switchcircuit 90 is coupled through the capacitor 139 to the transis tor 141in the switch circuit 140. Since the leading edge is rising, it onlyincreases conduction of the transistor 141, but does not afiect theoutput of the circuit 140. The trailing edge of the pulse 104 occurswhen the transistor 103 in the switch circuit 90 becomes conductive tocause current from the A+ supply voltage through the resistors 142 and144 to be diverted from the transistor 141, through the capacitor 139and the collector and the emitter of the transistor 103. When thecapacitor 139 becomes charged, the current will no longer becomediverted through the transistor 103, but, instead, will again bedelivered to the transistor 141 to render the same conductive. Thus, thetransistor 141 becomes nonconductive, as does the transistor 153. Therate of charge of the capacitor 139 is determined by the values of theresistors 142 and 144 and the capacitor 139. Thus, a positively-directedpulse 154 appears at the output of the fifth switch circuit 140, thatis, on the collector of the transistor 153. The leading edge of thepulse 154 occurs essentially simultaneously with the termination of thepulse 104. The capacitor 139 and the resistors 142 and 144 define a timeconstant network, the values of which control the duration of the pulse154.

The output of the fifth switch circuit 140 is coupled to a sixth switchcircuit 160 which has a construction identical to the switch circuit 50,with corresponding numerals, but with added thereto. Again, .forsimplicity purposes, only a portion of the switch circuit 160 isillustrated. When the collector of the transistor 153 in the fifthswitch circuit is at plus voltage for the duration of the pulse 154, thetransistor 161 and, thus, the transistor 173 are rendered conductive,effectively grounding the collector of the transistor 173, thusfurnishing a negatively-directed pulse 174 on the conductor 175. In aparticular form of the invention, the circuits 50, 70, 90, 110, 140, andconstituted a single integrated circuit.

The conductor 175 is coupled to a splitter circuit 180, which circuitincludes an isolating resistor 181 coupled to the base of a PNPtransistor 182. The emitter of the transistor 182 is coupled to the B+supply voltage, and the collector is coupled to the first oscillator190. There is provided another isolating resistor 184 coupled to theconductor 175 and a second PNP transistor 185 having its base coupled tothe resistor 184. The collector of the transistor 185 is coupled to thesecond oscillator 210. The negatively-directed pulse 174 renders thetransistors 182 and 185 conductive so as to furnish upon the respectivecollectors positively-directed pulses 186 and 187 having durations equalto the duration of the pulse 174.

Thus, when either of the switches 41 or 66 is actuated, the simultaneouspulses 136 and 137, each of the same predetermined duration, areproduced, followed automatically by a second pair of simultaneous pulses186 and 187, each of which lasts for another predetermined duration,there being virtually no time lag between the termination of the pulses136 and 137 and the commencement of the pulses 186 and 187.

The pulses 136 and 186 are coupled to the first oscillator 190 whichincludes a NPN transistor 191 having its base coupled through a resistor193 to the B+ supply and its emitter coupled through a resistor 192 toground reference potential. A pair of capacitors 194 and 195 is coupledin series between the collector of the transistor 19] and the B+ supplyvoltage. There is provided a connection between the emitter of thetransistor 191 and the junction of the capacitors 194 and 195. A coil196 couples the collector of the transistor 191 to the collectors of thetransistors 132 and 182. The junction of the capacitors 194 and 195 iscoupled through a variable resistor 197 and a capacitor 198 to anamplifier 200. The output of the amplifier 200 is coupled to the seriescombination of a capacitor 201 and a potentiometer 202 having a movablearm 203.

In the absence of the pulses 136 and 186, no supply voltage is furnishedfor the collector of the transistor 191, whereby the oscillator 190 doesnot produce an oscillatory signal or tone. The appearance of the pulse136 renders the oscillator 190 operative to produce an oscillatorysignal or tone for the duration of the pulse 136, which oscillatorysignal is coupled through the resistor 197 and the capacitor 198 to theamplifier 200. Upon the termination of the pulse 136, the pulse 186commences, as previously explained. Thus, the oscillator 190 ismaintained operative for the further duration of the pulse 186 toproduce the oscillatory signal or tone. The variable resistor 197enables adjustment of the amplitude of the tone.

The pulses 137 and 187 are coupled to a second oscillator 210 whichincludes an NPN transistor 211 having its base coupled through aresistor 213 to the B+ supply and its emitter coupled through a resistor212 to ground reference potential. A pair of capacitors 214 and 215 iscoupled in series between the collector of the transistor 211 and the B+supply voltage. There is provided a connection between the emitter ofthe transistor 211 and the junction of the capacitors 214 and 215. Acoil 206 having a plurality of taps 206.1 through 206.10 is coupled tothe collector of the transistor 211. In the embodiment shown, the tap206.5 is coupled to the collector of the transistor 135 and the tap206.8 is coupled to the collector of the transistor 185. The junction ofthe capacitors 214 and 215 is coupled through a variable resistor 217and a capacitor 218 to an amplifier 200.

In the absence of the pulses 137 and 187, no supply voltage is furnishedfor the collector of the transistor 211, whereby the oscillator 210 doesnot produce an oscillatory signal or tone. The appearance of the pulse137 renders the oscillator 210 operative to produce an oscillatorysignal or tone for the duration of the pulse 137, which oscillatorysignal has a frequency determined by the capacitors 214 and 215 and theportion of the coil 206 between the collector of the transistor 211 andthe tap 206.5. The oscillatory signal or tone is coupled through theresistor 217 which permits adjustment of the tone amplitude and thecapacitor 218 to the amplifier 200. Upon termination of the pulse 137,the pulse 187 commences, whereby the frequency of the oscillatory signalproduced by the oscillator 210 is determined by the same capacitors 214and 215 and a lesser inductance, that is, the portion of the coil 206between the collector of the transistor 211 and the tap 206.8. Theoscillatory signal produced for the duration of the pulse 187 is coupledby the resistor 217 and the capacitor 218 to the amplifier 200.

The amplifier 200 amplifies the various oscillatory signals or tonesapplied thereto and couples them via the capacitor 201 to thepotentiometer 202. Thus, there appears across the potentiometer 202 afirst tone having a frequency determined by the parts 194, 195, and 196simultaneously with a second tone having a second frequency determinedby the parts 214, 215, and the portion of the coil 206 to the tap 206.5.The second tone terminates with the termination of the pulse 137, butthe first tone is continuously produced by virtue of the pulses 136 and186. A third tone, having a frequency determined by the parts 214, 215,and the portion of the coil 206 to the tap 206.8, is produced during thepulse 187. There is provided, therefore, a continuous first tone, asecond tone, occurring simultaneously with the first portion of thefirst tone, and a third tone occurring simultaneously with the lastportion of the first tone.

It should be understood that the first oscillator 190 could be replacedwith an oscillator similar to the oscillator 210, so that the toneproduced thereby during the first pulse 136 differs from the toneproduced thereby during the second pulse 186. The circuit shown ispreferable because of its simplicity. By selecting the taps on the coil206 to which the transistors 135 and 185 are connected, the frequenciesof the tones may be determined. If used in a base station, a switch maybe connected between the transistors 135 and 185 and the taps on thecoil 206 to enable rapid selection of the tones. In a transmitteractually constructed, the duration of the tone from the oscillator 190was 325 milliseconds, the duration of the first tone from the oscillator210 was approximately 200 milliseconds, and the duration of the secondtone from the oscillator 210 was 125 milliseconds. All tones produced bythe oscillators 190 and 210 are in the voice spectrum, that is, betweenthe range of about 350 Hz to 2,750 Hz. The oscillator 190 produced atone at 2,450 Hz.

The tones were applied via the conductor 204 to the audio amplifier 121which couples the tones to the balanced modulator 22. The tones willactuate a specific receiver, as will be subsequently described. Thus, ifthe operator wishes to communicate with the specific receiver, theconnections to the coil 206 are made by switch, for example, and thenthe operator actuates the switch 41 or the switch 66, either of whichwould constitute a push-to-talk switch. That causes a pair ofsimultaneous tones immediately followed by a second pair of simultaneoustones to be impressed upon the balanced modulator 22. The tones, afterbeing modulated on a single side band, are transmitted to actuate theselected receiver. Since the tones are only sent for a very shortduration, in this example, 325 milliseconds, the operator may beginspeaking into the microphone 24 almost immediately, which voice messageis transmitted by modulation components on the single side band.

Turning now to FIG. 3, there will be described the details ofconstruction of the receiver used in the communication systemincorporating the features of the present invention. The receiver 240includes an antenna 241 which receives the signals emitted by thetransmitter 20 and applies them to an RF amplifier 242. The amplifiedsignals are applied to a convertor 243 having a second input coupled toa first oscillator 244. The RF signals from the amplifier 242 are mixedwith an oscillatory signal from the oscillator 244 to provide anintermediate frequency (1F) signal which is then applied to an IFamplifier 245. The output of the amplifier 245 is coupled to a productdetector 246, the latter receiving a second input from a secondoscillator 247. The second oscillator 247 reinserts the carrier whichwas suppressed at the transmitter 20 to detect the modulation componentsand apply them to an audio amplifier 248. The input to the audioamplifier 243 will consist of a first pair of simultaneous tones,followed immediately by a second pair of simultaneous tones, followed bythe voice message. The output of the amplifier 248 is coupled via thecontacts 249 of a relay 250 (which has an energizing winding 251) to aloud speaker 252. If the contacts 249 are open, no audio signal willarrive at the speaker 252 and, accordingly, no noise or information notdirected to the listener will be emitted therefrom.

The audio amplifier 248 is also coupled to a tone extractor 260. Thetone extractor 260 mixes the tones in a manner to be described, toprovide a signal having a frequency equal to the difierence of thefrequencies of the tones in the first pair, followed by a signal havinga frequency equal to the difference in frequencies of the tones in thesecond pair. The output of the tone extractor 260 is applied to adecoder 300 which will provide an output if the two signals appliedthereto are of the frequencies to which the decoder 300 is tunedv Theoutput of the decoder 300 is applied to an electronic switch 490 which,in the presence of a signal of the decoder 300, will furnish an enablingcurrent through the winding 251 of the relay 250, so as to close thecontacts 249. Audio signals produced by the amplifier 248 are thencoupled to the speaker 252 which converts them into sound waves.

Turning now to FIG. 4, the details of construction of the tone extractor260 will be described. As is usual, the audio amplifier 248 has atransformer 248a to match the impedance of the amplifier stages to theimpedance of the loud speaker 252. The secondary winding of thetransformer 248 is coupled in series with the loud speaker 252 and thecontacts 249 of the relay 250. A resistor is coupled across the primarywinding of the transformer 2480 to prevent the amplifier stages in theaudio amplifier 248 from oscillating when the contacts 249 are open, sothat those amplifier stages are unloaded.

The primary winding is also coupled to a detector or mixer 261 whichinciudes four diodes 262, 263, 264, and 265 arranged as a bridge to mixthe tones. A pair of oppositely-poled diodes 266 and 267 is coupled inparallel across the output of the mixer 261. There is provided alow-pass filter 270, also coupled to the output of the mixer 261, whichlow-pass filter 270 includes a number of stages. A resistor 271 andcapacitor 272 are coupled in parallel, providing the first stage. Thesecond stage is achieved in a T network defined by the resistors 273 and275 and the capacitor 274; a third stage in the form of an inductor 276and a capacitor 277; a fourth stage in the form of a capacitor 278 and aresistor 279 coupled in parallel; and a fifth stage consisting of aseries capacitor 280 and a shunt resistor 282. There is also provided astage of amplification in the form of an NPN transistor 281 biased bythe resistor to 8+. There is provided a second NPN transistor 287connected as an emitter follower, so as to match the impedance of thelow-pass filter 270 to the input impedance of the decoder 300. Furtherfiltering is provided by the tuned circuit consisting of an inductor 283and a capacitor 284, a series capacitor 285, and a shunt resistor 288.The output from the filter appears across an emitter load resistor 289and is coupled via a capacitor 290 to the decoder 300.

In operation, there appears across the secondary winding of thetransformer 248a a first simultaneous pair of tones which are mixed inthe mixer 261 to pro vide a signal having a frequency of one of thetones in the first pair, another signal having a frequency equal to thefrequency of the other of the tones in the first pair, still anothersignal having a frequency equal to the difference of frequencies of thetone in the first pair, and yet another signal having a frequency equalto the sum of the frequencies in the tones in the first pair. Similarly,there also appears across the second winding of the transformer 2480 asecond simultaneous pair of tones which are mixed in the mixer 261 toprovide a signal having a frequency of one of the tones in the secondpair, another signal having a frequency equal to the frequency of theother of the tones in the second pair, still another signal having afrequency equal to the difference of frequencies of the tones in thesecond pair, and yet another signal having a frequency equal to the sumof the frequencies in the tones in the second pair. The amplitudes ofthe signals from the mixer 261 are limited by the diodes 266 and 267.

The elements in the low-pass filter 270 are selected to cause the cutofffrequency thereof to be greater than the frequency equal to thedifferences in frequency between any pair of tones, but less than thefrequency of any individual tone. Thus, the cutoff frequency is lessthan the frequency equal to the sum of the frequencies of any twosimultaneous tones. Accordingly, the lowpass filter 270 passes only afirst signal having a frequency equal to the difference in frequenciesbetween the tones in the first pair, followed by a second signal havinga frequency equal to the difference in frequency between the tones inthe second pair. The two signals that are passed are amplified in thetransistor 281 and coupled by way of the emitter follower transistor 287and through the capacitor 290 to the decoder 300.

Turning now to FIG. 5, the details of the decoder 300 will be described.The decoder 300 includes an amplifier 301 which is coupled to thecapacitor 290 and having its output coupled to a tone filter 302. Thetone filter 302 includes capacitors 303 and 304 coupled in series and aninductor 305 coupled in parallel with the capacitor 304. The decoder 300further comprises a reference circuit 310 including an input capacitor311 coupled to the output of the amplifier 301 and a diode 312 coupledto ground. There is also provided a diode 313 connected to the junctionof the capacitor 311 and the diode 312. A filtering network comprises aresistor 314 and a capacitor 315 coupled in parallel to ground. There isalso provided a rectifying circuit including a pair of diodes 316a and317 coupled in series to the base of a switching transistor 318. Acapacitor 319 is coupled between the junction of the capacitors 303 and304 and the junction of the diodes 316a and 317. There is also provideda resistor 320 and a capacitor 321 for filtering of the rectifiedvoltage, the resistor 320 also providing a DC return for the base of thetransistor 318. The transistor 318 is connected as an emitter follower,the emitter being coupled to a load resistor 322 connected to groundreference potential. The emitter of the transistor 318 is coupled by wayof a capacitor 323 to an NPN transistor 324, the emitter of which isgrounded and the base of which is coupled to the B+ supply voltage byway of a resistor 325.

There is also provided a second filter circuit 332 which includescapacitors 333 and 334 coupled in series and an inductor 331 coupled inparallel with the capacitor 334. Also, the reference circuit 310includes a second diode 316b which is coupled in series with a diode 347to the base of a switching transistor 348. A capacitor 349 is coupledbetween the junction of the capacitors 333 and 334 and the junction ofthe diodes 316b and 347. There is also provided a resistor 350 and acapacitor 351 for filtering of the rectified voltage, the resistor 350also providing a DC return for the base of the transistor 348. Thetransistor 348 is connected as an emitter follower, the emitter beingcoupled to a load resistor 352 connected to ground reference potential,

the collector being coupled to the B+ supply voltage. The base of thetransistor 348 is also coupled back to the collector of the transistor324.

Prior to reception of any tones, the transistor 324 is conducting byvirtue of the forward bias provided by the current flow through theresistor 325. Thus, the base of the transistor 348 is effectivelygrounded to prevent amplification of a tone thereby. If tones arereceived, they are applied to the amplifier 301, which amplifier hassufficient gain to cause the tones therefrom to be clipped or limited sothe signal strength at the antenna 241 does not affect the amplitude ofthe signals applied to the filters 302 and 332.

The amplified signal from the amplifier 301 containing the tones andnoise will be filtered in the reference circuit 310 and will berectified thereby to provide a reference voltage on the anode of thediode 316a. If the signal from the amplifier 301 includes the tone towhich the filter 302 is tuned, the filter 302 will develop its maximumvoltage which is applied to the cathode of the diode 316a. In order thatthe diode 316a may conduct to provide an output, the tone appearing atthe cathode thereof must have a peak-to-peak value in excess of thereference voltage on the anode of the diode 316a. The rectified voltage,after being filtered by the resistor 320 and thecapacitor 321, isapplied to the base of the transistor 318 so as to render it conductive.Current flows from the B+ supply through the collector and the emitterof the transistor 318, through the capacitor 323 and the base-emitterjunction of the transistor 324. Since the transistor 324 is alreadyconducting, the presence of the tone has little effect. When the firstsignal terminates because of termination of the first pair of tones, thecapacitor 323 discharges through the resistor 322 to rendernonconductive the transistor 324, thereby removing the short on the baseof the transistor 348. The length of time the transistor 324 isnonconductive, and, therefore, the length of time the short is removedfrom the transistor 348 is determined by the time constant of theresistor 322 and the capacitor 323 and the resistor 325. However, untilthe correct second signal is received, the transistor 348 is notrendered conductive.

When the first pair of tones terminates, the second pair of tonescommences and if the frequency thereof is the frequency to which thefilter 332 is tuned, the filter 332 will develop its maximum voltagewhich is applied via the capacitor 349 to the cathode of the diode 316b.In order to provide an output from the diode 348, the signal appearingat the cathode of the diode 3l6b must have a peak-to-peak value inexcess of the reference voltage on the anode of the diode 3l6b. Thesignal is rectified by the diodes 3l6b and 347 which, in effect,constitute a doubler circuit and filtered by the resistor 350 and thecapacitor 351. If the short on the base of the transistor 348 furnishedby the transistor 324 has been removed, then the rectified voltagerenders the transistor 348 conductive to cause the B+ voltage(approximately) to appear on the emitter of the transistor 348. Ifdesired, a feed-back network may be provided from the transistor 348 tothe transistor 324 to maintain the latter nonconductive for the durationof the second tones. Alternatively, the time constant determined by theresistor 322 and the capacitor 323 and resistor 325 may be selected toinsure that the transistor 324 is not conductive throughout the durationof the second tones.

There is provided an electronic switch 400, which, in the embodimentshown, is a monostable multivibrator and includes an NPN transistor 401having its emitter coupled to ground via a resistor 402 and having itsbase coupled to ground by way of a resistor 403 and a capacitor 404coupled in parallel. There is also provided a PNP transistor 405 havingits base connected directly to the collector of the transistor 401, itscollector connected through a resistor 406 to ground and its emitterconnected to the source of supply voltage, a resistor 407 beingconnected between the base and the emitter of the transistor 405. Thecollector of the transistor 405 is coupled by way of a capacitor 408 anda diode 410 to the base of the transistor 401. A diode 411 is coupledbetween ground reference potential and the junction of the capacitor 408and the diode 410. The emitter of the transistor 348 in the decoder 300is coupled to the base of the transistor 40]. A diode 415 couples thecollector of the transistor 405 to a conductor 416. There is provided aswitch 417 coupled between the source of 3+ operating potential and theemitter of the transistor 401. The switch 417 is also coupled via adiode 418 to the conductor 416.

In operation, the appearance of the output signal on the emitter of thetransistor 348 causes conduction of the transistor 401 which provides apath for current flow from the source of supply voltage through thebase-emitter junction of the transistor 405 and the collector and theemitter of the transistor 401. This renders the transistor 405 highlyconductive so as to provide current flow through its collector and itsemitter and the resistor 406 and thereby cause conduction of the diode415 to place the supply voltage on the conductor 416. The supply voltagebecomes an enabling signal for causing current flow in the winding 251of the relay 250 to close the contacts 249. The capacitor 404 must becharged before the transistor 401 will conduct. Thus, the capacitor 404introduces a slight delay to prevent the electronic switch 400 fromproducing the enabling signal in the presence of a static charge. Theisolating diode 410 prevents the signal from the decoder 300 from beingapplied to the capacitor 408. The diode 411 provides a rapid dischargepath for the capacitor 408.

During the conduction period of the transistors 401 and 405, currentflows from B+ through the collector and the emitter of the transistor405, through the capacitor 408 and through the base-emitter junction ofthe transistor 401 to charge the capacitor 408. Accordingly, when thesignal from the decoder 400 is removed by virtue of the tonesterminating, the transistor 401 remains conductive because the capacitor408 has a charge thereon, which charge leaks off through thebase-emitter junction of the transistor 401 and the resistors 402 and403. Of course, the conduction of the transistor 401 maintains thetransistor 405 conductive to maintain the enabling voltage on theconductor 416 for a time interval determined by the RC time constant ofthe switch circuit 400, that is, the resistors 402 and 403 and thecapacitor 408. By selecting the value of those parts, the time periodthat the enabling signal remains on the conductor 416 may be controlled.

With the relay 250 energized, audio signals from the audio amplifier 248will be applied to the loud speaker 252 for conversions into soundwaves. It is thus desirable that the RC time constant in the electronicswitch circuit 400 be selected to be long enough to maintain thecontacts 249 closed for the duration of audio information. The switch417' is provided to enable the user to override the timing function byrendering nonconductive the transistors 401 and 405. The B+ supplyvoltage is then directly applied through the diode 418 to energize therelay winding 251, whereby the contacts 245 will remain closed as longas the switch 417 is energized.

Summarizing, if the operator of the transmitter 20 wishes to alert theuser of the receiver 240, the operator connects the transistor to a tapon the coil 206 in the oscillator 210 such that the difference infrequency between the tones produced by the oscillator 190 and theoscillator 210 is the frequency to which the filter 302 is tuned. Thetransistor is connected to a tap on the coil 206 such that thedifference in frequency between the tone produced by the oscillator andthe tone produced by the oscillator 210 is equal to the frequency towhich the tuned circuit 332 is tuned. By actuating his push-to-talkswitch, which may be either of the switches 41 or 66, the operator willcause the tones to be modulated as single side-band components. Thetones are received by the receiver 240, processed by elements 242, 243,245, and 246 and applied to the audio amplifier 248. The mixer 261 firstmixes the first pair of tones (the tone produced by the oscillator 190and the tone first produced by the oscillator 210) and thereafter mixesthe second pair of tones (the tone produced by the oscillator 190 andthe tone next produced by the oscillator 210), to provide sum anddifference frequency signals for each. The low-pass filter 270 allowsonly signals representative of the dif ference in frequency between eachpair of tones to be applied to the decoder 300. Since the first tunedcircuit 302 is tuned to the frequency of the first signal, thetransistor 324 will be rendered nonconductive to remove the short on thebase of the transistor 348. Since the second signal has a frequencycorresponding to that to which the tuned circuit 332 is tuned, thetransistor 348 will be rendered conductive to provide an output signalfor application to the electronic switch circuit 400. The electronicswitch circuit 400 in response thereto will produce an enabling signalon the conductor 416 which extends beyond termination of the last pairof tones in the sequence of tones, the duration beyond termination beingdetermined by the RC time constant of the circuit 400. The enablingsignal energizes the relay 250. Since the relay 250 becomes energizedduring the presence of the last pair of tones, the user of the receiver240 will hear the same from the loud speaker 252. This is advantageousin that it alerts the user of an impending message. All of this takes avery short period of time, for example, 350 milliseconds, so that theoperator of the transmitter 20, immediately upon actuation of thepush-to-talk switch, may speak. The voice message is processed in thereceiver and is coupled to the loud speaker 252 since the relay 250 isenergized.

As previously pointed out, the tones produced by oscillators 190 and 210in the transmitter 20 have frequencies within the voice spectrum, thatis, within the range of about 350 Hz to 2,800 Hz. This is desirable.first to minimize the portion of the frequency spectrum required by anyindividual communication system. Because the tone frequencies are withinthe voice spectrum, the extent of the frequency spectrum needed isminimized. Secondly, utilizing tones only within the voice spectrumnarrows the passband to which the receiver 240 must respond. Basically,the narrower the passband of the receiver, the better thesignal-to-noise ratio thereof.

The transmission of a pair of simultaneous tones compensates for anydrift that may occur either in the frequencies of the tones themselvesor in the suppressed carrier. Thus, for example, if the first tone is2,500 Hz and the second tone is 1,800 Hz, the difference will be 700 Hz.If a downward 100 Hz frequency drift occurs, so that the first tone hada frequency of 2,400 Hz, the second tone would similarly drift to 1,700Hz, and the difference would still be 700 Hz.

Although the voice spectrum is about 350 Hz to about 2,800 Hz, whichwould therefore be the required passband for the receiver 240, the tonesshould be within a narrower range, for example, 500 Hz to 2,650 Hz, inorder to compensate for up to 150 Hz drift. The optimum number of tonefrequencies would be obtained if the cutoff frequency of the low-passfilter 270 would be about one half of the uppermost frequency.Therefore, if the uppermost frequency was 2,650 Hz, the theoreticaloptimum cutoff frequency of the lowpass filter 270 would be 1,325 I-Iz.If, as is preferable, one tone in each pair of tones always has the samefrequency of, say, 2,650 Hz, then the other tones should be selectedfrom within the range of 1,325 Hz (the cutoff frequency of the low-passfilter 270) and 2,150 l-Iz (assuming the lower end of the receiverpassband is 500 Hz). Of course, because the filter 270 is not perfect,the cutoff frequency may have to be somewhat less than 1,325 Hz. Byselecting tones from within the range of 1,350 Hz and 2,150 Hz, alldifference frequency signals would fall within the range of 500 Hz to1,350 Hz.

It should be understood that the instant communication system enablesthe tones to have frequencies within the voice spectrum and yet notinterfere with the voice message because the tones are transmittedbefore the voice message commences and an electronic switch circuit hastiming means to render operative the audio circuit beyond termination ofthe tones.

In one form of the invention the tone extractor 260 had the followingparts: diodes 262, 263, 264, 265, 266, and 267 were silicon diodes;resistor 271 was 220 kilohms; capacitor 272 was 5.5 microfarads;resistors 273 and 275 were 22 kilohms; capacitor 274 was 0.047microfarads; inductors 276 and 283 were 3 henries; capacitors 277 and278 were 8.2 microfarads; resistor 279 was kilohms; capacitors 280 and285 were 0.01

microfarads; resistor 282 was 5.6 kilohms; capacitor 284 was 0.147microfarads; resistor 288 was 100 kilohms; resistor 289 was 470 ohms;capacitor 290 was 33 microfarads.

In order to maximize the number of tones available for a selectivecalling system, it may be desirable to add a multiplier circuit to theoutput of the tone extractor 260, which multiplier may be of a standardconstruction. Preferably, the multiplier would be a frequency doubler,so that, if, for example, the frequency were 1,000 Hz, the multiplierwould produce a signal with a frequency of 2,000 Hz. Thus, if the spreadof frequencies from the tone extractor 260 were 500 Hz to 1,35 0

Hz, the doubler would increase the frequency range from 1,000 Hz to2,700 I-Iz; that is a 1,700 Hz spread, as opposed to an 850 Hz spread.This would effectively increase the number of tones which can betransmitted and received in the system.

It is believed that the invention, its mode of construction andassembly, and many of its advantages should be readily understood fromthe foregoing without further description, and it should also bemanifest that, while preferred embodiments of the invention have beenshown and described for illustrative purposes, the structural detailsare, nevertheless, capable of wide variation within the purview of theinvention, as defined in the appended claims.

What is claimed is:

1. In a communication single side-band transmitter, the combinationcomprising a first oscillator for generating a first tone, first meansfor producing a first pulse of limited duration, said first oscillatorbeing coupled to said first means and responsive to said first pulse toproduce said first tone for the duration of said first pulse, a secondoscillator having frequencydeterrnining elements and being operative toproduce second tones having frequencies respectively in accordance withthe frequency-determining elements in circuit with said secondoscillator, second means for producing a sequence of a plurality ofsecond pulses respectively of limited durations, said second oscillatorbeing coupled to said second means and responsive to said second pulsesto produce a sequence of a plurality of second tones respectively forthe durations of said second pulses, means coupled to the outputs ofsaid first and second oscillators for combining the tones therefrom andthereby provide a sequence of a plurality of second tones and acontinuous first to'ne simultaneously with said plurality of secondtones.

2. In the communication single side-band transmitter of claim 1, saidsecond means being operable to produce a sequence of two second pulsesand said second oscillator being operative in response to said secondpulses to produce a sequence of two second tones.

3. In the communication single side-band transmitter of claim 1, saidfirst tone and the first of said second tones commencing atsubstantially the same time, and said first tone and the last of saidsecond tones terminating at substantially the same time.

1. In a communication single side-band transmitter, the combinationcomprising a first oscillator for generating a first tone, first meansfor producing a first pulse of limited duration, said first oscillatorbeing coupled to said first means and responsive to said first pulse toproduce said first tone for the duration of said first pulse, a secondoscillator having frequency-determining elements and being operative toproduce second tones having frequencies respectively in accordance withthe frequency-determining elements in circuit with said secondoscillator, second means for producing a sequence of a plurality ofsecond pulses respectively of limited durations, said second oscillatorbeing coupled to said second means and responsive to said second pulsesto produce a sequence of a plurality of second tones respectively forthe durations of said second pulses, means coupled to the outputs ofsaid first and second oscillators for combining the tones therefrom andthereby provide a sequence of a plurality of second tones and acontinuous first tone simultaneously with said plurality of secondtones.
 2. In the communication single side-band transmitter of claim 1,said second means being operable to produce a sequence of two secondpulses and said second oscillator being operative in response to saidsecond pulses to produce a sequence of two second tones.
 3. In thecommunication single side-band transmitter of claim 1, said first toneand the first of said second tones commencing at substantially the sametime, and said first tone and the last of said second tones terminatingat substantially the same time.